nLab smooth topos

Idea

A smooth topos or smooth lined topos is the kind of topos studied in synthetic differential geometry, a category of generalized smooth spaces for which a notion of infinitesimal space exists.

It is defined to be a category of objects that behave like spaces, one of which — the line object $R$ — is equipped with the structure of a commutative algebra, such that for infinitesimal objects $S \subset R^n$ all morphisms $S \to R$ are linear — i.e. such that the Kock-Lawvere axiom holds.

Definition

There is a standard definition and various straightforward variations.

Standard definition

For $(\mathcal{T},R)$ a lined topos there is the notion of an $R$-algebra object in $\mathcal{T}$. For $A$ and $B$ any two $R$-algebra objects, let $R Alg_\mathcal{T}(A,B)$ denote the subobject of $B^A$ consisting of morphisms $A \to B$ which are algebra homomorphisms. Let $Spec(A)$ denote the algebra spectrum $R Alg_\mathcal{T}(A,R)$ of $A$ in $\mathcal{T}$.

Definition

An $R$-Weil algebra $W$ is an $R$-algebra of the form $W = R \oplus J$, where $J$ is an $R$-finite-dimensional nilpotent ideal.

Definition

A lined topos $(\mathcal{T},R)$ is a smooth topos if, for each $R$-Weil algebras $W$, the functor $(-)^{Spec W} : \mathcal{T} \to \mathcal{T}$ has a right adjoint and the canonical morphism

$$W \to R^{Spec(W)}$$

is an isomorphism in $\mathcal{T}$.

That is to say, each $R$-Weil algebra in a lined topos is infinitesimal and satisfies the Kock-Lawvere axiom. The right adjoint of $(-)^{Spec W}$ is known as the “amazing right adjoint”.

Examples

Well-adapted models

A smooth topos $(\mathcal{T},R)$ is called a well adapted model if there is a full and faithful functor

$$Diff \hookrightarrow \mathcal{T}$$

from the category Diff of smooth manifolds into it, that takes the real line $\mathbb{R}$ to the line object $R$.

In these well adapted models ordinary differential geometry is therefore faithfully embedded.

For a list of examples of well adapted models see

• Models for Smooth Infinitesimal Analysis.

Notice that by far not all models are of this form, as the following examples show. On the contrary, the axioms of synthetic differential geometry may be regarded as providing a unified framework in particular for differential geometry of manifolds and algebraic geometry of algebraic spaces, schemes and other objects.

Models for supergeometry

It is straightforward to slightly enhance the axioms of smooth toposes such as to incorporate the step from differential geometry to supergeometry, one just requires that algebra structure on the line object $R$ is further refined to that of a superalgebra. The result is called a super smooth topos. See there for a list of models of these.

Models for algebraic geometry

A simple model of a smooth topos that may be regarded as a context inside which much of algebraic geometry takes place is the following:

Let $k$ be a field and let $(k-Alg^{finp})^{op}$ be the opposite category of the category of finitely presented $k$- algebras. Then the presheaf category $\mathcal{T} = PSh((k-Alg^{finp})^{op})$ equipped with the line object $R = k[T]$ (the algebra of polynomials over $k$ in one variable $T$)

$$( \mathcal{T} = PSh((k-Alg^{finp})^{op}), R = k[T] )$$

is a smooth topos. This is described in section 9.3 of

• Anders Kock, Synthetic geometry of manifolds (pdf)

Notice that despite the name of that book, this model is not a well adapted model in that ordinary smooth manifolds do not embed full and faithfully into this topos.

Instead, interpreting the internal notion of manifold described in that book – called formal manifolds in the model $\mathcal{T} = PSh((k-Alg^{finp})^{op})$ produces something like formal schemes over $k$.

Indeed, much of algebraic geometry over $k$ may be thought of as being concerned with this model for a smooth topos. A main difference is that in algebraic geometry attention is usually focused on particularly well behaved objects inside $\mathcal{T} = PSh((k-Alg^{finp})^{op})$: those that satisfy a sheaf condition with respect to a Grothendieck topology on $(k-Alg^{finp})^{op}$ and among those moreover those that are locally isomorphic to a representable: these are the schemes or algebraic spaces over $k$.

Probably the category of sheaves localization of $PSh((k-Alg^{finp})^{op})$ with respect to one of the standard topologies (Zariski or etale) is still a smooth topos, so that this condition can be enforced by passing to a more restrictive model.

But since schemes alone will never form a topos, and smooth topos axiomatization of algebraic geometry will always contain more general – also more “pathological” – objects than schemes. It’s an example of an old dictum by Grothendieck that it is more useful to have a nicely behaved category that contains pathological objects than a badly-behaved category of only nice objects: the topos-general nonsense allows useful general constructions in $\mathcal{T}$ which may in each individual case be checked for whether they land in the sub-category of schemes or algebraic spaces or not.

A similar comment of course applies to the “well-adapted models” mentioned above: into these ordinary manifolds only embed, they contain “smooth space”s much more general than manifolds (such as diffeological spaces) but also possibly “pathological” ones, from some perspective or other.

See also

• synthetic differential geometry applied to algebraic geometry.

Warning on preservation of (co)limits

Most of the examples above provided toposes into which a category $NiceSpaces$ of nicely behaved spaces, such as manifolds or schemes embeds full and faithfully.

$$NiceSpaces \hookrightarrow \mathcal{T} \,.$$

But the inclusion functor will in general not preserve all limits and colimits that exist in $NiceSpaces$.

For instance for the well-adopted models the full and faithful inclusion of Diff typically respects only pullbacks of manifolds along transversal map. This is because this is the case for the inclusion $Diff \hookrightarrow \mathbb{L}$ of Diff into the category of smooth loci and the Grothendieck topology for these models is typically subcanonical. See the discussion at smooth locus for more on this.

This means that universal constructions in a smooth topos may yield different results than in a smaller category of more nicely behaved spaces. However, it is noteworthy that in the above example of manifolds, one may argue that the ordinary pullback of manifolds along non- transversal maps is the wrong pullback in any case: it doesn’t behave well with the cohomology of manifolds. One motivation for derived geometry, in the case of manifolds specifically the motivation for considering derived smooth manifolds, is to pass from objects in a topos instead more generally to objects in an (∞,1)-topos of stack ∞-stacks.